Contractility is a fundamental property of almost all types of cells. A solution of the basic mechanism of the acto-myosin interaction is thus essential to any understanding of the movement of cells, the control of cell shape and perhaps even cell-cell interactions. Our approach to this problem has been to isolate the individual contractile proteins, and characterize their structural and functional properties by a combination of chemical, physical-chemical and immunological techniques. Specific goals are to discover the role of the low molecular weight subunits in myosin; the effect of phosphorylation on the acto-myosin system; the effect of calcium on the regulatory activity of myosin from vertebrate skeletal and smooth muscles; and the contribution of proteins other than myosin to the assembly and function of the thick filament. A comparison of the properties of smooth muscle myosin with those of non-muscle myosins, may reveal whether these myosins are alike in their mode of assembly and interaction with actin. The distribution of isoenzymes of myosin during the course of development will be followed by an immuno-cytochemical approach. A correlation of specific myosin isoenzymes with the physiological and enzymatic characteristics of a muscle may lead to new insights into how the myosin molecule works.